1. Field of the Invention
The present invention relates to a fine surface profile measuring apparatus for measuring surface roughness of, e.g., various metal products or mirrors or a surface pattern of, e.g., a semiconductor chip or optical disk on an order of nanometers with high accuracy using an optical method.
2. Description of the Related Art
Conventional optical size measuring apparatuses using a critical angle prism are described in Japanese Patent Disclosure (Kokai) Nos. 59-90007, 60-38606, 61-240103, and 58-176505. These apparatuses are examples of a measuring apparatus using a critical angle prism in accordance with a focusing error method. That is, a reflection amount of the critical angle prism varies in accordance with whether a sample is located in front of or behind a focal point of an objective lens. Therefore, the above apparatuses detect the reflection amount and measure the height of the sample.
Japanese Patent Disclosure (Kokai) No. 59-27207 describes a non-contact surface profile/roughness meter for measuring a displacement in accordance with a vertical movement designation signal of an objective lens in a focusing apparatus by vertically moving an objective lens in accordance with a focusing error detection signal.
Although any of the above apparatuses measures a fine area of the sample, there is no optical system for observing the surface of the sample. Therefore, a point of a sample to be measured is not accurately determined. Especially when a sample has a fine structure as an IC wafer, although a measurement point must be positioned on a micron order, this operation cannot be performed. In addition, since an error is produced in a measurement value when dust or a scratch is present on a measurement point, the state of a surface must be checked.
An example of a conventional apparatus which eliminates the above drawbacks is a microdisplacement measuring microscope described in Japanese Patent Disclosure (Kokai) No. 62-36502. This apparatus has a microscope for observing the surface of a sample in an enlarged scale. In this apparatus, a single objective lens is commonly used as an objective lens for the microscope and that for a displacement measurement optical system.
In this apparatus, however, since the objective lens is commonly used for the two optical systems, complexity of optical axis adjustment is amplified when the objective lens is to be exchanged in accordance with resolution of displacement measurement or a measurable displacement range. In this case, although measurement of the apparatus is performed in nanometers, no countermeasure for vibration is made. Therefore, the measurement is susceptible to a vibration of a flat or the vibration of an air. When an observer moves closer to an eyepiece to use the microscope, the optical systems slightly vary by an air flow caused by breathing of the operator, thereby varying a measurement value.
Furthermore, a sample is scanned generally by a pulse motor. However, scanning is temporarily stopped at each measurement point because the pulse motor is step-driven. In addition, measurement is started after vibration caused by an impact upon stopping is sufficiently suppressed, resulting in time-consuming measurement.